1. Field of the Invention
This invention relates to techniques for detecting magnetic ink characters on documents and recognizing them. In particular, the invention relates to the technology of suppressing fluctuations in magnetic ink character detection signals caused by suboptimal printing of magnetic ink characters.
2. Description of the Related Art
FIG. 9 shows a schematic view of a conventional magnetic ink character detection apparatus. In FIG. 9, magnetic head 1 is used as a magnetic signal detection unit, and a DC motor 11 is used as a drive source for transporting the medium on which magnetic ink characters are printed. The medium is transported by a transport mechanism composed of a transmission belt 10 and a rubber roller 6. When a magnetic ink character printed on the medium passes over magnetic head 1, magnetic head 1 converts the magnetic flux generated by the magnetic ink character into an electrical signal.
Generally, the scanning height for a magnetic head is provided in the direction in which the medium is transported. The scanning height is designed to be wider than the height of the magnetic ink characters. Prior to the detection processing, the magnetic ink characters are remagnetized to a specified polarity. This causes the magnetic head to output an electrical signal converted from a magnetic flux representing a change in the height component (also referred to as the vertical component) of a given magnetic ink character.
The shapes of magnetic ink characters are standardized under ISO/R1004, so that fixed electrical signal waveforms are produced by a predetermined transport speed of the medium. FIG. 5 shows an example of magnetic ink characters that are printed on medium 8. Magnetic ink characters may be printed in either of two fonts: E13B and CMC7. The font shown in the FIG. 5 is E13B. The signal waveform shown in the lower portion of the figure represents a change as a function of time of the electrical signals that are output from amplification circuit 2, wherein the magnetic ink character detection apparatus reads the medium in FIG. 5 from "0" to "3".
As the signal waveform indicates, the signal shows a positive peak based on a change in the vertical component of a magnetic ink character, at the local maximum of the change ratio in the vertical component. Similarly, the signal shows a negative peak at the local minimum of the change ratio in the vertical component. The magnitude of a peak depends upon the rate of increase or decrease of the vertical component.
When detecting a magnetic ink character by transporting a medium, the apparatus first amplifies the electrical signals that have been output by the magnetic signal detection unit, detects the positive and negative peaks, and then it determines the peak positions on the time axis. The apparatus can detect a magnetic ink character by determining that the positive peak position of the first waveform as the beginning of a given character, and by detecting the presence or absence of the positive and negative peaks of waveforms at a fixed interval that is determined by the transport speed of the medium.
When a magnetic ink character has been detected, the character is recognized by the positions at which the positive peaks and the negative peaks of the signal waveforms that are associated with the characters have been pre-stored as pattern data. The pattern data associated with magnetic ink characters are synchronized with specific starting positions on the basis of detected signal waveforms, and the magnetic ink characters are recognized by referencing the pattern data.
In the related art described above, however, in cases where the conditions under which magnetic ink characters are printed in different conditions between one medium and another, the positive and negative peak values of the electrical signals that are output from the magnetic signal detector can vary from one medium to another. Specifically, if the magnetic ink contains a high concentration of a magnetic material, or the character height is substantially increased because of an increase in the width of lines composing the characters due to print smudging (hereinafter referred to as a "positive scattering"), the signal peaks that are output by the magnetic head increase. Conversely, if the concentration of magnetic material is low or the character height is substantially decreased due to a decrease in line width (hereinafter referred to as a "negative scattering"), the signal peaks that are output by the magnetic head decrease.
If the magnetic ink characters that are printed exhibit a positive scattering, the electrical signals that are output are amplified to a greater amplitude by the succeeding amplification circuit. If the amplified electrical signal, after amplification, is greater than the maximum output voltage of the amplification circuit, i.e., if the electrical signal exceeds the dynamic range of the amplification circuit, the electrical signal waveform exhibits a saturation state as shown in FIG. 6. In the figure, the area indicated by Tpk1a, Tpk1b, Tpk1c and Tpk1d represent a saturated area. Thus, a problem in the related art is that in a saturated condition, the position at which the peak of a waveform occurs can be indeterminate.
By contrast, if a printed magnetic ink character scatters negatively, the signal that is output from the magnetic head decreases. Consequently, the signal amplitude that is output by the amplification circuit also decreases, as shown in FIG. 7. As indicated by the broken lines in FIG. 7, normally a signal waveform contains an overlapping external noise, such as magnetic noise. Therefore, if the position of a peak in the signal waveform is detected by using a conventional peak detection method, the peak detection position fluctuates in the range indicated by Tpk2 in FIG. 7.
Therefore, in the aforementioned conventional recognition method that uses peak positions of the detection signals for the recognition of characters, the inability to accurately determine peak positions results in an decrease in the recognition probability of magnetic ink characters.